1. Field of the Invention
This invention concerns a smoke filter in general, and more specifically relates to structure for a smoke filter especially adapted for use in an internal combustion engine exhaust system.
2. Description of the Prior Art
A very effective type of smoke filter has been found to be the catalytic type which employs alumina coated wire, such as steel wool or the like, as a substrate for the alumina. However, a key difficulty with such type of filter has been found to be the loss of resilience of the substrate material which occurred with exposure to high temperatures for extended periods of time. The grain growth of the wire, or steel wool, resulted in its becoming a brittle support structure for the alumina coating. And the coating does not add any strength of its own. Consequently, the mechanical vibrations and thermal gradients, which result in expansion and contraction of the substrate filter elements relative to one another, create difficulties in maintaining the physical shape such that it will maintain necessary resilience and so an adequate surface for contacting the hot exhaust gases without channeling.
Furthermore, while a recent U.S. Patent (which is assigned to the same assignee as this application) i.e. No. 4,039,294, shows structure for a similar filter, it is clearly a relatively expensive type of mechanical structure. Also, it does not suggest the use of a tapered housing, so that is is unable to obtain the additional benefits of having radial compressive force applied to the filter medium.
Consequently, it is an object of this invention to provide structure that is low cost and yet effective in overcoming difficulties heretofore encountered with the filter medium for a particular type of smoke filter.